Technical Abstract:
Following decades of chicken litter applications to soils of the Delmarva Peninsula, legacy phosphorus (P) is a major source of soluble P entering drainage ditches that eventually empty into the Chesapeake Bay. In April, 2007, flue gas desulfurization (FGD) gypsum, also called synthetic gypsum, was used to construct a ditch filter on the University of Maryland Eastern Shore’s research and teaching farm at Princess Anne, MD. Soluble P in ditch flow that passed through the bed of FGD gypsum precipitated as calcium phosphate; the filter achieved a mean removal efficiency of 75%. This presentation evaluates environmental concerns surrounding the slightly higher levels of mercury (Hg) and arsenic (As) in FGD gypsum than in naturally occurring mined gypsum under conditions of extreme leaching. Filtered water had no detectable Hg at detection limit of 1 ug L** -1, indicating that Hg does not leach from the gypsum at concentrations that might cause concern. Although arsenate, which is present in elevated levels in poultry litter - amended soils, has similar chemical behavior as phosphate, the gypsum filter is not effective at reducing soluble As in ditch drainage waters. Concentrations of soluble As in influent and effluent samples are similar. This is likely a result of competition between P and As in sorption and precipitation reactions, coupled with the much higher concentrations of phosphorus found in the ditch influent as compared to arsenate concentrations. The gypsum filter also acts as a sediment trap for particulate-bound P. Aqua regia digests of samples taken from the surface (0 to 2 cm) of the gypsum bed in December, 2010 showed elevated levels of Hg and As in comparison to the concentrations of these metals in the FGD gypsum used to construct the filter. When these accumulations are taken into consideration, the gypsum filter is a net sink for Hg and As.